Ceria-based catalysts

Active heterogeneous catalysts have been obtained. Examples include titania-, vanadia-, silica-, and ceria-based catalysts. A survey of catalytic materials prepared in flames can be found in [20]. Recent advances include nanocrystalline Ti02 [24], one-step synthesis of noble metal Ti02 [25], Ru-doped cobalt-zirconia [26], vanadia-titania [27], Rh-Al203 for chemoselective hydrogenations [28], and alumina-supported noble metal particles via high-throughput experimentation [29]. 

Daturi, M., Bion, N., Saussey, J. et al. (2001) Evidences of a lacunar mechanism for deNO, activity in ceria-based catalysts, Phys. Chem. Chem. Phys., 3, 252. 

A marked effect of the Ce02/Zr02 composition (in samples containing 40 wt.% NiO) on the catalytic activity was noticed. The catalysts with Ce Zr =1 1 (6A) were not only more active (than 7A and 8A) but were also stable during the reaction. Sample 8A containing no zirconia in the support showed a low activity. The NiO crystallite size (Table 11.2) in these compositions varied in the order 7A < 6A < 8A. It may be recalled that on ceria-based catalysts the crystallite size of nickel metal was similar to that of NiO. The higher activity for 6A than 7A indicates that in addition to accessibility of... 

Ceria-based catalysts [219] and Ce02 Zr02 solid solution catalysts [220] have each been reported to be very efficient catalysts for the synthesis of EC and PC by reaction of C02 with ethene glycol and propene glycol, respectively. The catalytic activity has been shown to depend heavily on the composition and calcination temperature of the catalysts. 

Three major effects were revealed during this high-throughput study of this set of close reactions on ceria-based catalysts (Fig. 10.16) ... 

Fig. 10.16 Comparison of Pt-ceria-based catalysts performances (a, b) CO conversion, (c) OSC, as a function of temperature (°C) under various operating conditions.

R. Valencuela, G. Bueno, A. Solbes, Nanostructured ceria-based catalysts for oxydehydrogenation of ethane with CO2, Topics Catal. 15 (2-4) (2001) 181-187. 

Other methods The traditional methods, such as gas condensation, sol-gel methods, precipitation with postheating methods, and mechanical milling methods are widely used for the synthesis of nano-crystalline ceria materials and ceria based catalysts (Fu et al., 2001 ... 

The selective oxidation or preferential oxidation of CO in hydrogen-rich stream is another important object for ceria based catalysts. The gas mixture from steam reforming/partial oxidation of alcohols or hydrocarbons, followed by the WGS reaction contains mainly FI2, CO2 and a small portion of CO, H2O, and N2. When such gaseous stream would be taken as input for hydrogen fuel cells, the CO has to be removed to avoid poisoning of the anode electrocatalysts. Ceria based nanomaterials, such as ceria/gold, ceria/copper oxide catalysts exhibit suitable catalytic activities and selectivities for CO PROX process. 

Other oxidation/reduction related reactions are also explored with ceria based catalysts. For example, Murugan and Ramaswamy (2007) reported the oxidative dehydrogenation of ethylbenzene on nanocrystal-line ceria using N2O as the oxidant Concepcion et al. (2004) reported the chemoselective hydrogenation of crotonaldehyde catalyzed by Ft on mesostructured Ce02 NPs embedded within layers of Si02 binder. 

The defective structure in nanocrystalline ceria based catalysts proved to have strong effect on the OSC. Mamontov et al. (2000) reported the neutron diffraction studies of the atomic structures of nanocrystalline powder of ceria and ceria-zirconia solid solution. They found that the concentration of vacancy-interstitial oxygen defects has a direct correlation with the OSC. This effect is stronger than the correlation of surface area with OSC. Zirconia reduces ceria and preserves oxygen defects to retard the degradation of ceria-zirconia in OSC. Yan et al. observed the strong correlation between OSC and the lattice strain in nanosized ceria-zirconia, which could be measured via XRD (Si et al., 2004 Figure 11). 

Ceria-based catalysts are intensively used because of their high chemical and physical stability, high oxygen mobility and high oxygen vacancy concentrations, which are characteristic of fluorite-type oxides. The possibility of cycling easily between reduced and oxidized states (Ce Ce" ) permits the reversible addition... 

Oxygen Storage/Redox Capacity and Reiatcd Phenomena on Ceria-Based Catalysts ... 

A wealth of experimental chemisorption data are presently available for NM/Ce(M)02-, catalysts. As reported in section 4.3.2.2, the increase of generally induces significant modifications on their chemical behaviour. In most of cases, partial rather than complete inhibition of their chemisorption capability is reported. In many cases, however, the techniques and/or experimental routines do not allow an unequivocal interpretation of the reported H(CO)/NM data. As noted in section 4.3.2.2, quite often, the role played by a number of very important side effects, like the metal or support sintering, the adsorption of the probe molecules (H2 and CO) onto the supports, the presence of chlorine in them, or the reversibility of the deactivation phenomena, has not been established. By contrast, there are a number of recent studies (97,117,163,235) from which meaningful conclusions may be drawn. Moreover, some of them (97,163) have provided some additional fine details about the nature of the metal/support interaction effects occurring in ceria-based catalysts. 

In ceria-based catalysts, one main aspect studied with XPS is the cerium redox state (i.e. the proportion of Ce and Ce " ) for different preparations or treatments. Peak u, located at Eb==916.5 eV, which appears to be typical of and absent for Ce, and consequently decreases upon reduction, has been used frequently with this... 

X-ray Absorption Fine Structure (XAFS) spectroscopic studies of ceria-based catalysts concentrate frequently on the Ce Lm-edge, in which electrons from the Ce... 

OXYGEN STORAGE/REDOX CAPACITY AND RELATED PHENOMENA ON CERIA-BASED CATALYSTS... 

To date the surface science approach and techniques such as those described above have been used to study structure of ceria surfaces, the adsorption and desorption of several molecular species on ceria and model ceria supported catalysts, and the co-adsorption and reaction of certain of these molecular species. The results provide a basis for clarifying the elementary reaction steps underlying catalytic processes occurring on ceria based catalysts. In this Chapter it is attempted to review and summarize this research. 

Of major concern for ceria based catalysts is the reducibility of the ceria support. Several papers mention experiments related to this subject. For model surfaces it is possible to quantitatively determine an average oxidation state of the ceria film, i.e. the ratio of Ce / Ce This is most commonly done using XPS of the Ce 3d photoemission features. The 3d spectrum is complicated by satellite features which are well described and provide useful variation with Ce oxidation state. ... 

For use in catalytic combustion where high temperature are encountered, where huge amount of water and carbon dioxide are produced, all ceria based catalysts must meet the following requirements ... 

In ceria-based catalysts, one main aspect studied with XPS is the cerium redox state (i.e. the proportion of Ce and Ce " ) for different preparations or treatments. Peak u, located at Eb 916.5 eV, which appears to be typical of Ce and absent for Ce, and consequently decreases upon reduction, has been used frequently with this aim. The percentage %(u ) of the total integrated spectrum intensity (after baseline subtraction) lying within this peak in pure, nonreduced Ce02 has been computed to amount to 13.7% [108]. The %(u ) value of an analyzed material is frequently used in the literature to quantify its redox state, assuming e.g. that the the quotient betwen it and the value for Ce02 gives the fraction of Ce in the material. It has been however pointed out in early works [109], that this value is not proportional to the amount of Ce present specifically Romeo et al. [110] claimed that peaks v... 

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